Obstacle detecting system



Patented July 9, 1946 OBSTACLE DETECTING SYSTEM Clarence D. Tuska,l Philadelphia, Pa., assignor to Radio Corporation of Americma Delaware corporation of Application July"3`0,'1938, Serial No. 222,088l

4 Claims.

This invention relates to obstacle detection and more specifically to a system in which pulses of radio frequency energy'- are di-rectively radiated to be reflected by an object to be detected.

Obstacle detecting systems using radio frequency energy have been proposed. In one system, a pulse of radio frequency energy is radiatedy toward an object and simultaneously with the radiation a cathode ray is deected along one path. After the pulse of radio frequency energy has been reflected by an obstacle, it is applied toV areceiver which is connected to deflecting electrodes ofthe cathode raytube so. that the. raymay be deected at. an angle to its first mentioned path. If the cathode ray traverses the rst mentioned path at a uniform rate dependent upon the range of the obstacle detector and startingV in. synchronism with. each outgoing pulse, thereceived reflected signals willV caues the second mentioned deections at points proportionalto the distance froml the transmitter to the reflected object.

One of the difficulties with systems. of this type is that the indication of the reflections is pro-y portional to the time required for the radio Wavel to travel to the object and back tothereceiver and such indications are independent of direction. With such systems, the observer is unable to distinguish between indications resulting from reected Waves arriving at different. angles.. For example, if one reflection is caused by an object due north of the observer a distance of 1500 meters an exactly similar irniication will result from an object 1500 meters northeast of the observer. One obvious correction of this` difficulty is to make the radiated Waves as directiveas possible. If the obstacle detector is to be used in a xed position Where suflicient space: is available, it is possible to erect an antenna. array whichv will provide the required degree of directivity.

If the obstacle detecting system is to be used on a relatively small vehicle suc-h as an airplane, it becomes very difficult to erect an antenna system which will have the required directivity and will not at the same time put unreasonable drag on the craft. Perhaps an obvious solution oi the antenna array for aircrafty would be the utilization of extremely high frequencies which would permit the use of a reflector for focusing or beaming the Waves. While this solution is practical from the standpoint of the, antenna. itv requiresv the generation of Waves having a frequency of the order of 3,000,000,000 cycles per second: While currents of such frequencies are generated in the laboratory, it does not. seem entirely prac.-l ticaI toy generate, amplify and detect radio frefluency. oscillations ofthe above mentioned :Ere-

quency in any practical installation on an aircraft.

It is an object of this invention, therefore, to

,obtain the required directivity for obstacle detection byvutizing pulses of radio frequencyenergy having a frequencyl of 'ther orderV of 500 megacyclesper second and utilizing directive antennas of suchy construction as are practical on airplanes. It is afurther object of this invention to provide means whereby radio frequency fields navi-ngen' overlappingportion of the required directivity for obstacleV detection areestabl-ishedf. Itis a stillv further object toA provide means for directively* and simultaneously radiating pulses of rad-io fre-Y quency energy of,v4 different frequency indifferent directionsV whereby the simultaneous reception` of these Waves operates a receiver responsive to the difference in frequency of the pulses. A still further obj ect of the invention is to providev` means for distinguishing indications obtained by reflections from obstacles locatedv on the desired coursefrom objects to the-left lorrig-ht ofthe course.

The invention will bev described by referenceitot 25 the accompanying drawing in lwhich which maybe dipoles or the like.

Figure 1 isa schematic illustration of one e bodiment ofthe invention, and

Figures 2 and 3 are-graphicillustrations used in describing the invention.

Referring to Fig. 1, a keyer l is connected to The Vkeyer may also be connected either directly or through the network l5to a receiver I9. The input of the receiver i9 is connected to a receiving antenna 2l which may'include a reector 23. In sornel installations either or both ofthe transmitting antennas 1, 9 may be used as thereceiving antenna. The output of the receiver is connected to a second pair ofV defiecting electrodes; of the cathode ray tube I1; l

The principle of operation is as follows: The keyer l generates pulses: which are sharply defined and of an interval of the lorder of a fraction of a micro-second. These pulses are simultaneously applied to the pulse generators of frequency f1 and f2 whereby the generators are keyed on for a brief interval. The on interval is followed by an off interval which is long enough to permit the outgoing waves to travel to a distant object and return before the next pulse of radio frequency energy is radiated. The two transmitting antennas 1, 9, together with their reflectors II, I3 are arranged to direct the radiation so that the major axes of the field patterns are directed toward the obstacles to be detected along the course of the moving craft carrying the apparatus.

The transmitted pulses of radio frequency energy f1 and f2 are radiated toward a distant 0bject to be reected from the object back to the receiving antenna 2| which may be made directive along the desired course. The received waves induce currents in the directive antenna 2| which are applied to any suitable means for combining the two currents to derive a current of a frequency equal to the difference in frequency of the transmitted pulses. For example, if the transmitted pulses have a frequency f1 and f2, respectively, the pulses, after reiiection, will be combined in the receiver to generate currents of frequency f1-fz. These currents fi-fz may be amplified and demodulated. The demodulated currents are impressed on the second pair of deflecting electrodes of the cathode ray tube I1. Since the receiver is only responsive to currents of the diiference frequency fi-fz, it follows that the receiver will only respond when the two transmitted pulses, after reflection, simultaneously arrive at the receiver. In the event that one impulse for example f1, arrives before the second impulse f2, there will be no current of difference frequency f1-f2 produced and therefore no signal will appear in the output of the receiver.

It should be understood that the receiving antenna may be suitably shielded to prevent direct pickup from the transmitting antennas. In the event that the shielding is insufficient and to prevent any residual currents from blocking the receiver, it is preferable to apply an initial biasing impulse, which may be derived directly or indirectly from the keyer I, to make the receiver inoperative for the brief instant at which the radiation takes place from the transmitter. It should also be understood that the receiver may utilize a conventional radio frequency detector and audio frequency circuits.

In Fig. 2 is illustrated an aircraft 25 from which pulses of radio frequency energy having a field pattern 21 are radiated. The course of the plane, with a factor of safety on either side thereof. is indicated by the dash lines 29. In the event that objects are present at points 3i, 33 and 35, it Will be seen that the system would be responsive to reflections from all three objects. These reflections on the conventional system would all appear as obstacles to be avoided by the operator of the aircraft. However, it follows, from inspection, that the objects represented by the reference numerals 3i and 35 are suiiiciently oi the course as to represent no obstacle in the path of the airplane. With respect to the object indicated by the reference numeral 33, this would represent an obstacle to be avoided.

While the foregoing system has the characteristics shown in Fig. 2, the system with this invention has the characteristics represented by Fig. 3. In this illustration pulses of radio frequency energy f1, fz having field patterns 39, 4I, respectively, are simultaneously radiated from the aircraft 31. It will be seen that these field patterns have a common or overlapping portion 43 which sharply defines the path of the craft as represented by the dash lines 45. In the event that there are reflecting objects as indicated by the reference numerals 41, 49, 5I, it follows that reflections from 41 will only contain energy of frequency f1 while reections from the object represented by reference numeral 5I will only contain energy of frequency f2. Since these objects are located at different distances from the plane, the reflected energy will not arrive simultaneously and will not cause any undesired indication. If, however, an obstacle represented by reference numeral 49 lies in the overlapping portion of the fields, the reflected pulses f1 and f2 will reach the receiver simultaneously and will, therefore, cause an indication. While the system of the invention thus appears to limit itself to obstacles directly on the course of the plane, it will appear that two objects located at equal distances, although not on the desired course, may cause reiiections to be simultaneously received and indicated. Such a situation is probably unlikely and would be transitory instead of giving a continuous indication.

Thus. it appears that the present system simultaneously radiates impulses of two different radio frequencies along slight different paths having a common field portion. The transmittedY pulses are refiected and if the reflections of the two freduencies arrive simultaneously at the receiver. they are combined to produce an indication. AIn

the event that the pulses arrive at different times,

they are not combined and no indication is obtained. In generalthe simultaneous reflection will only occur in the common portion of the overlapping iields. No attempt has been made to describe the kever, pulse generators, cathode ray tube or deiiecting voltage. network. as these elements are well known to those skilled in the,

means for receiving said pulses after reflection,

from said obstacle, said receiving means being responsive to currents of the difference frequency of said rst mentioned frequencies and4 being unresponsive unless said currents of both frequencies are simultaneously applied.

2. In a system of the character of claim 1 a cathode ray tube including ray deflecting means, means for deecting said ray in synchronism with said keying, and vmeans for indicating onA said cathode ray tube the simultaneous reception of said signals.

3. 'I'he method of detecting objects by means of pulses of radio energy which includes radiating pulses of radio frequency energy in different directions, each of said pulses comprising atrain of radio frequency waves, adjusting directivity of said radiations to establish a common field, de

riving currents from the reflection of said pulses, combining said currents upon simultaneous re.

ception, demodulating said combined currents, and indicating the presence of a pulse reflecting quency when said simultaneous pulses are simultaneously received, deriving from said combined currents a current whereof the frequency is equal to the difference between the frquencies of the combind currents, demodulating said different frequency current, and indicating the presence of a pulse reflecting object in said com-y mon portion as a function of said demodulated currents.

CLARENCE D. TUSKA. 

